perf(ws): optimize message handler lookup

[CodyTseng]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

Each WebSocket request requires iterating through all message handlers to find the appropriate one now. When there are many message handlers, this is much less efficient than using a Map for lookup.

What is the new behavior?

Using a Map for message handler lookup instead of traversing an array.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 4f50b2b4-f390-4601-a1d4-4d83c3c20299

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.123%

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Totals
Change from base Build eacd3e56-4bed-4f5e-9bab-412a10335aab:	0.0%
Covered Lines:	6736
Relevant Lines:	7312

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💛 - Coveralls

[kamilmysliwiec]

lgtm